1. Field of the Invention
The invention relates generally to methods for preparing and storing embryonic plant tissue and its subsequent use in regeneration and transformation. The invention relates generally to methods for preparing and storing embryonic plant tissue directly from dry seed and its subsequent use in regeneration and transformation. The resulting novel explant is storable and can germinate and or be transformed when appropriate conditions are provided.
2. Description of Related Art
Transformed plants may be obtained by directly treating meristematic tissue of a plant embryo (e.g. U.S. Pat. No. 6,384,301). The meristematic tissue contains formative plant cells that differentiate to produce multiple plant structures including stem, roots, leaves, germ line tissue, and seeds. Plant embryos may be treated and selected or screened to determine which of those treated embryos have incorporated the new genetic information into germ line tissue. U.S. Pat. Nos. 6,384,301 and 7,002,058 and U.S. Publication 20060059589 describe methods of genetically transforming soybeans (Glycine max) using bacterial-mediated gene transfer directly on the meristematic cells of soybean embryos.
In typical soybean transformation procedures, seeds are hydrated/imbibed to soften the seed coat and allow for or extraction of the explant tissue. After hydration the embryo or embryonic tissue is excised from seed. When meristems are used as the explant, primary leaf tissue may be removed to expose the meristem of the soybean embryo. Considerable effort is involved in excising the embryos, transferring the genetic material into the embryos, and culturing the embryos. Processing can cause damage to the explant tissue, which negatively impacts subsequent transformation and regeneration steps. It is thus important to reduce damage to the explant tissue that could result in transformation and/or regeneration effort being applied to non-viable tissue.
The excision of plant embryos is often therefore performed by hand. In this process, surface sterilized seeds are aseptically handled one at a time with gloved hands. The explant is then carefully excised. In the case of meristems, the seeds are carefully oriented in a manner as to eject the seed coat with applied force and then the embryonic leaves (cotyledons) are removed near the primary meristem to leave the seed embryo containing meristematic tissue. Even the careful handling of individual seeds, however, results in less than desirable recovery of viable embryos, and may be less than 70% even with high quality seeds.
Bacterial contamination of embryos after excision is also a significant concern. The increased handling to preserve higher viability and recovery of explants also increases the likelihood of destructive contamination (which will manifest itself in subsequent processing steps). Such contamination can result in significant loss, as a single contaminated explant will contaminate other samples during tissue culture. This causes loss of yield and/or transformation frequency, and eventually transformation efficiency. Moreover, the manual excision is extremely labor intensive, time-consuming, and stands as a barrier to a scaling up of the transformation process in which many plants must typically be treated to yield desired results.
In addition, the current processes are limited because the harvested explant must be moved quickly into the subsequent steps of transformation, or viability is lost. Typically, once an explant is harvested (e.g. U.S. Patent Application Publication 20050005321 and corresponding PCT Publication WO 2005/000471), it is placed on media and subjected to co-culture with transforming bacteria within hours of being removed. Thus, anytime transformations are to be performed, it is necessary to first prepare all of the explants that will be needed so they are ready to immediately move into an incubation or transformation process. This timing can be very complicated and inflexible, particularly if sudden demands arise and explants cannot he prepared in time. Clearly, the lack of an ability to store such explants for more than a few hours is a major deficiency in the art.
There remains a great need for processes that can increase the availability of transformable embryos without unacceptably increasing total costs and/or timelines of explant preparation for transformation. The ability to store excised meristematic tissue (explant) for later use in particular has been lacking. Such methods would substantially increase the availability of transformable embryos, and allow efficient planning and execution of large-scale transformation studies. Such methods should enable explant storage to meet demands created during peak operational hours or during inadvertent disruptions in the production line and shipping of explants, for example, to different sites for maintaining production runs. Further, the use of dry embryo explants (artificial seeds) for transformation is not known in the art due to common wisdom in the art to use “wet” explants for transformation.